Spin-down rate of the transitional millisecond pulsar PSR J1023+0038 in the optical band with Aqueye+

We present a timing analysis of the transitional millisecond pulsar PSR J1023+0038 using observations taken between 2018 January and 2020 January with the high time resolution photon counter Aqueye+ mounted at the 1.82 m Copernicus telescope in Asiago. We report the first measurement of the timing solution and the frequency derivative of PSR J1023+0038 based entirely on optical data. The spin-down rate of the pulsar is (−2.53 ± 0.04) × 10−15 Hz2, which is ∼20 per cent slower than that measured from the X-ray observations taken in 2013–2016 and ∼5 per cent faster than that measured in the radio band during the rotation-powered state.